This invention relates to electrical resistance elements and to the method of making such elements, and more specifically to printed resistors used in trimmers or potentiometers or other devices, where a series of variable linear resistances or a variable, non-linear resistance is desired.
Variable resistance elements are either of the linear type wherein, the resistance is a direct function of the position of the electrical contact member, or wiper, on the resistance element, or the non-linear type wherein the resistance is a non-linear function of the contact position. Non-linear elements have been made by several methods, as for example, constructing a resistance track of molded conductive plastic of varying thickness, irregularly shaping a conductive section forming a resistance track, or constructing electrical shunts along a resistance track. One of more common types of non-linear elements is constructed of two different resistance materials along the resistance track. Each separate material has a linear relationship of resistance to wiper position, however, each material is selected to more closely approximate a desired non-linear function than could be provided with a single resistance material. The disadvantages to this type of arrangement are that the resultant output does not sufficiently reduce the difference between the actual resistance and the desired resistance, there is a noise spike at the point where the two elements join.
In devices using the type of element having two or more resistance materials which change linearly with the change in wiper position, the resistance element is usually formed of two materials such as cermet, deposited in line along the element track. The wiper is moved along the surface of the element to change the resistance. As the wiper moves along the element surface the total resistance in the circuit follows a linear rate of change until the wiper encounters the second resistance material. With the wiper in contact with the second material the change of the resistance is still linear but the rate of change is different from the rate of change when the wiper was in contact with the first material. In this way a non-linear output of the device is approximated. To more closely approximate a desired non-linear output, it has been necessary to deposit additional, different, resistance materials. The present invention allows a non-linear rate of change or numerous linear rates of change of resistance to be accomplished with two resistance materials and only one firing of both resistance materials. Other methods of producing non-linear outputs have been attempted, as for example producing a second parallel low resistance material adjacent to the resistance track and tailoring the low resistance track to the output desired. See, for example, U.S. Pat. No. 3,379,567. This type of device is essentially a multiple shunt device.